Study of spin, parity and couplings of a bosonic resonance at colliders[HBNI Th92]

Abstract

In this thesis we outline a step by step methodology to determine the spin, parity and couplings of the 125 GeV resonance (H) discovered at LHC. We study experimentally clean “gold-plated” decay mode of H, where it decays to two Z bosons followed by subsequent decays of the two Z bosons into four non-identical charged leptons. Since the resonance is found to be decaying into di-photon channel i.e. H→ γ γ it can not be a Spin-1 particle, prohibited by Landau-Yang's theorem. We begin with writing down the most general Lorentz and gauge invariant vertex factor of H for both the spin possibilities i.e. Spin-0 and Spin-2. We then derive three uniangular distributions in terms of several angular asymmetries. These angular asymmetries have definite parity signatures and are orthogonal to each other. Thus they can be measured independently. We find that these asymmetries have different characteristics for a Spin-0 resonance compared to a Spin-2 resonance and can be used to determine the spin of H. After identifying the spin, these asymmetries can be used to study the parity of H. We corroborate our methodology by numerical analysis.

In SM the value of the HZZ couplings are uniquely predicted. After the discovery of H, it is esstial to study the HZZ couplings precisely to confirm its SM nature. Although 8 TeV run indicates H to be a Spin-0 resonance with even parity, a considerable amount of CP-odd admixture is still possible. We show in this thesis, how one can precisely probe CP-odd admixtures of HZZ couplings at 14 TeV LHC run for two different luminosities. The direct measurement of the absolute values of the couplings are beyond the scope of the LHC which requires a precise measurement of the partial rate. We show how using angular asymmetries one can obtain the ratios of couplings and the relative phases between them. This precision measurement of couplings have the potential to find New Physics beyond SM.

Almost all GUT motivated theories predict the existence of a massive Z' boson which is Spin-1 boson. Finding such particle would give us unique understanding of the physics beyond the Standard Model. In this thesis we show how three uniangular distributions can be used to probe the spin, parity and couplings of a Z' via gold-plated decay mode where it decays to two Z bosons. A numerical analysis is also performed to show the applicability of our methodology for experimental study.

Finally we have combined these results and presented a step by step methodology to uniquely determine the spin, parity and couplings of a new resonance via gold-plated decay mode with any arbitrary mass. Thus it would not be a overstatement that uniangular distributions have the potential to confirm the true characteristic of bosonic a resonance.